1. Field of the Disclosure
This disclosure generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to a method and apparatus having the flexibility to provide an image of a borehole wall for a wide range of borehole sizes.
2. Background of the Art
Electrical borehole logging is often used to provide images of an electrical property of boreholes. There are two categories of devices typically used as electrical logging devices for imaging boreholes. In the first category (galvanic devices), a measure electrode (current source or sink) is used in conjunction with a return electrode (such as the tool body). A current flows from a current source in the tool to a measure electrode through the earth formation. The current returns to the source via another electrode. The second category relates to inductive measuring tools in which an antenna within the electrical logging tool induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate antenna.
There are several modes of operation of a galvanic device. In one mode, the current at a current electrode is maintained constant and a voltage is measured between a pair of monitor electrodes. In another mode, the voltage of the measure electrode is fixed and the current flowing from the measure electrode is measured.
The galvanic devices are contact devices, in that the measure electrodes generally come in contact with the borehole wall during logging of the wellbore. Such devices are sensitive to the effects of borehole rugosity, borehole size, and the standoff of the pad carrying the measure electrodes. The present disclosure provides an improved electrical logging tool that reduces the effects of the pad offset.